This invention relates to an X-ray generator, and in particular to apparatus for prolonging the life of an X-ray target used within an X-ray generator.
Known X-ray generators comprise an electron gun, an X-ray target and an X-ray exit window. These generators produce X-rays by accelerating electrons from the electron gun into the x-ray target. X-rays are emitted from the target through the exit window. Such generators may be in the form of sealed X-ray tubes, for example microfocus tubes, which are evacuated once and then sealed off, or in the form of rotating anode generators, which are permanently connected to vacuum pumps and are continuously evacuated during operation.
A major limitation to the longevity of X-ray generators is the lifetime of the target. All targets degrade over time due to the effects of heat and roughening caused by the electron bombardment. There are various known methods for reducing these effects, including cooling the back of the target with flowing water or rotating the target so that no one area of the target is continuously subjected to the electron bombardment. Methods of increasing the cooling efficiency have been proposed based on using high conductivity materials such as diamonds. However, these methods are not in common usage currently.
With known X-ray generators, it can take a number of minutes after switching on the machine before it has stabilized and is ready for use. As a result, many generators are simply left running throughout the day, so that the xe2x80x9cwarm-upxe2x80x9d or stabilization delay is removed. This means that the electrons are focussed on the target for long periods of time during each use of the generator, which leads to accelerated degradation of the target, even though the radiation produced by the X-ray generator is used only for short periods.
In cases where the construction of the generator permits, the target can be replaced. Where the construction does not permit target replacement in a routine procedure, then it is common practice to discard the complete tube assembly making up the X-ray generator.
In commercially available sealed tube and rotating anode generators, there is no provision to control the position of the beam on the target or to control the quality, size or shape of the focal spot on the X-ray target. The quality of the X-ray beam emitted can deteriorate rapidly with prolonged use due to contamination and damage to the target area under continuous electron bombardment.
In the case of rotating anode generators, once performance has degraded below a useful level, replacement of the target is required. This entails cost of replacement parts as well as significant down time of the generator. In the case of sealed tube generators it is necessary to discard the whole tube and replace it with a new tube.
It is an object of the present invention to provide means to lengthen the life of a target, and thereby to lengthen the life of the X-ray generator. By controlling the position and brightness of the beam, the apparatus according to the present invention can reposition and modify the area of focus of the beam. Defocussing the beam reduces the flux per unit area of electrons on the target. Repositioning the beam enables a fresh area of the target to be exposed to electrons. The lifespan of the target is prolonged by either of these means, and the time interval between replacements of the target or of the complete tube assembly is increased.
A consequence of the approach of the present invention is that the tube is only required to run in operational condition with the target exposed to focussed electrons when the operator requires the X-ray beam to be produced.
According to the present invention, there is provided an X-ray generator comprising an electron gun, electron focussing means, a target and electronic control means, wherein the area of the target on which the focussing means causes electrons from said electron gun to impinge comprises an X-ray source, the control means being adapted to control the electron focussing means so that the X-ray source on said target may be varied in size and/or shape and/or position.
According to a first aspect of the invention the control means includes a switching means to switch the electron focussing means between a first unfocussed state in which the X-ray source has a first area and a second focussed state in which the X-ray source has a second area smaller than said first area. The second area may be a line, a spot or some other profile. The first area may be a line of greater thickness, a spot of greater diameter or some other shape.
Preferably said first area has a surface area at least twice, more preferably four times, most preferably ten times that of said second area.
According to a second aspect of the invention the control means includes a switching means to switch the electron focussing means between a plurality of focussed states, whereby in each state the X-ray source is in a corresponding discrete position on said target. The X-ray source may be in the form of a line, a spot or some other profile on the target.
The electron gun may comprise an evacuated tube around which the electron focussing means is mounted outside the vacuum. Alternatively the electron gun may comprise an evacuated tube within which the electron focussing means is mounted. The evacuated tube may be a sealed vacuum tube or may be connected to a vacuum pump which permits continuous evacuation during operation of the generator.
The electron focussing means may comprise an x-y deflection system for centering the electron beam in the tube. The electron beam focussing means may further comprise at least one electron lens, preferably an axially symmetric or round lens, and/or at least one quadripole or multipole lens for focussing the electron beam to a line focus and for steering the electron beam.
The electron beam lenses may be magnetic or electrostatic.
Preferably the target is metal, most preferably a metal selected from the group Cu, Ag, Mo, Rh, Al, Ti, Cr, Co, Fe, W, Au. The target surface may be orientated such that the plane of the target surface is perpendicular or at an angle to the axis of the X-ray tube.
According to a third aspect of the present invention there is also provided a method for extending the life of a target of an X-ray generator, wherein the generator comprises an electron gun, electron focussing means and a target, the method comprising the steps of:
firing electrons at the target such that the area of the target on which the focussing means causes electrons from said electron gun to impinge comprises an X-ray source,
controlling the electron focussing means to move between a first unfocussed state in which the X-ray source has a first area and a second focussed state in which the X-ray source has a second area smaller than said first area, the intensity of electron impingement in the first state being sufficiently low to reduce target degradation, the intensity of electron impingement in the second state being sufficiently high such that the source produces a predetermined required level of brightness and source size on the target. The source may be a spot, a line or some other profile.
Preferably the electron beam current is substantially the same in the first and second states, while the intensity of the beam per unit area at the target is lower in the first state than in the second state.
According to a fourth aspect of the present invention there is provided a method for extending the life of a target of an X-ray generator, wherein the generator comprises an electron gun, electron focussing means and a target, the method comprising the steps of:
firing electrons at the target such that the area of the target on which the focussing means causes electrons from said electron gun to impinge comprises an X-ray source,
controlling the electron focussing means to move between a plurality of focussed states, whereby in each state the X-ray source is in a corresponding discrete position on said target, such that the intensity per unit area in each discrete position is substantially constant, and such that there is no overlap on the target between the discrete positions corresponding to each focussed state. The source may be a spot, a line or some other profile.
The lack of overlap between the discrete positions on the target means that a fresh area of target is used as a source each time the electron focussing means moves to a new state. The control of the electron focussing means may be manual but is preferably electronic, so that each discrete position corresponds to a pre-programmed control signal applied to the electron focussing means.